Comprehensive study of the effects of rolling resistance on the stress–strain and strain localization behavior of granular materials

This paper presents the results of a comprehensive study of the effects of rolling resistance on the stress–strain and strain localization behavior of granular materials using the discrete element method. The study used the Particle Flow Code (PFC) to simulate biaxial compression tests in granular materials. To study the effects of rolling resistance, a user-defined rolling resistance model was implemented in PFC. A series of parametric studies was performed to investigate the effects of different levels of rolling resistance on the stress–strain response and the emergence and development of shear bands in granular materials. The PFC models were also tested under a range of macro-mechanical parameters and boundary conditions. It is shown that rolling resistance affects the elastic, shear strength and dilation response of granular materials, and new relationships between rolling resistance and macroscopic elasticity, shear strength and dilation parameters are presented. It is also concluded that the rolling resistance has significant effects on the orientation, thickness and the timing of the occurrence of shear bands. The results reinforce prior conclusions by Oda et al. (Mech Mater 1:269–283, 1982) on the importance of rolling resistance in promoting shear band formation in granular materials. It is shown that increased rolling resistance results in the development of columns of particles in granular materials during strain hardening process. The buckling of these columns of particles in narrow zones then leads to the development of shear bands. High gradients of particle rotation and large voids are produced within the shear band as a result of the buckling of the columns.     

Effect of processing parameters on shear bands in non-isothermal hot forging of Ti -6Al -4V

Abstract

The alloy system Ti- 6Al- 4V is the prominent Ti alloy system for aerospace and biomedical applications, as a result of its mechanical property balance and biocompatibility. Since the mechanical characterisation of Ti- 6Al- 4V is strongly sensitive to processing parameters there is relationship between processing variables, i.e. strain rate and temperature, microstructure, and properties under different loading conditions. Two phase (α + β) titanium alloys undergo flow instabilities and are susceptible to shear bands or regions of localised deformation crossing many grains during hot forging under non-isothermal conditions (dies and workpiece at different temperatures). Under such conditions shear bands can be generated even in materials without flow softening attributes. This occurs if the forging parameters lead to large amounts of heat transfer between the dies and the workpiece. This study investigates the occurrence of shear bands during non-isothermal, hot forging of Ti -6Al- 4V in order to evaluate the process parameters that generally lead to shear bands in conventional hot forging of metals. Upset compression tests on cylindrical specimens were conducted in a mechanical press and lateral side pressing tests on long, round bars were performed in either a mechanical press or a hydraulic press. The tests ranged from axisymmetric to plane strain compression. In upset specimens shear bands occurred at an angle of 45° to the compression axis and bands of intense deformation separated chill zones from the deforming bulk. Observation also demonstrated that the fracture might be owing to microvoids nucleated at weak points in sections of the shear surfaces. For plane strain deformation, shear bands were found to initiate along zero extension directions in a manner analogous to the formation and propagation of shear bands in isothermal hot forging. Although the shear band features at hot forging temperatures were similar to each other, there was a difference in the hardness and thickness of the shear bands depending on deformation mode, amount, and temperature.     

The Bond Mechanism in Stone- or Brick-to-Grout Interfaces

Abstract:  An experimental research on the bond between masonry units and grout has been carried out at the National Technical University of Athens. In this study, tension and shear tests on composite substrate/grout specimens were performed. The composite specimens were consisted of two pieces of substrate connected with one grout joint. The substrate was either stone or brick, whereas hydraulic lime or tripartite (lime–pozzolan cement) or cement grouts were used. The experimental results have demonstrated that the developed tensile and shear bond strength of the studied hydraulic lime and tripartite grouts is comparable with those of cement-based grouts. Moreover, the value of the reached bond strength (tensile or shear) is governed mainly by the substrate characteristics and the binding properties of the grouts. It was also found that interfaces subjected to shear exhibited similar behaviour with interfaces within concrete; thus the beneficial effect of normal compressive stress on the interface was confirmed. Finally, the experimental results are used for the formulation of a Mohr–Coulomb criterion for grout to travertine joints in shear–compression and shear–tension. The results of this project clearly confirm the efficiency of grouts with medium or low mechanical properties (tripartite and hydraulic lime grouts) for retrofitting historic masonries.     

Computational assessment of ballistic impact on a high strength structural steel/polyurea composite plate

Ballistic impact on a polyurea retrofitted high strength structural steel plate is simulated and validated. A soft material model for polyurea, which is capable of capturing complex mechanical behavior characterized by large strains, hysteresis, rate sensitivity, stress softening (Mullins effect), and deviatoric and volumetric plasticity, is calibrated against several uniaxial tension experiments and a three-dimensional release wave experiment to capture both the material point and bulk behaviors. A porous plasticity model is employed to model the high strength structural steel and localization elements are included to capture adiabatic shear bands and strain localization. The computational capabilities of these models are demonstrated by the prediction of the target plate displacement, which shows excellent agreement with experiments.     

Interventions to historic masonries: Investigation of the bond mechanism between stones or bricks and grouts

Within the present work, the mechanism of bond is studied in composite grout/substrate specimens. Three types of tripartite (lime-pozzolan-cement) grouts are examined, combined with three substrates (two types of limestone and bricks). The interfaces between grout and substrate are characterized by means of mechanical tests in direct tension and shear. The in-time development of the tensile and shear bond strength is also investigated. In order to correlate the characteristics of the substrates and the obtained strengths of the interfaces, their surfaces and porosity are examined. The main conclusions of this study are that the studied tripartite grouts can develop tensile and shear bond strength comparable to Portland cement-based grouts, and that the value of the reached bond strength is governed mainly by the substrate characteristics and the binding properties of the grouts. The results of this project confirm the efficiency of tripartite lime-pozzolan-cement grouts with reduced Portland cement content for repair and strengthening of historic masonries.     

Orientation-dependent compression/tension asymmetry of short glass fiber reinforced polypropylene: Deformation,damage and failure

Short glass fiber reinforced polypropylene (sgf-PP) is increasingly employed in structural components which are subjected to a variety of loading conditions including tensile, compressive and bending loading modes. Since typical industrial components exhibit a wide range of fiber orientation distributions, their mechanical response to these loading conditions is also highly anisotropic. In this paper, the compression/tension asymmetry in the stress–strain behavior of sgf-PP is investigated from a macroscopic engineering and a micro-mechanisms of deformation and failure point of view for specimens with varying, precisely defined fiber orientations. Furthermore, we performed volume strain measurements and two-cyclic tests. We used the results to deduce the onset of damage due to cavitational mechanisms under tension and compared this to the onset of deviation of the tensile from the compressive stress–strain behavior. The results showed a good correlation for specimens with high fiber orientation, whereas for specimens with low fiber orientation results deviate due to the high deviatoric matrix volume strain contribution.     

Cavity mediated strain localization and overall ductility in eutectic tin-lead alloy

A combined experimental and numerical study is undertaken to examine the effects of pre-machined holes on strain localization and overall ductility in eutectic tin-lead alloy. Thin-sheet specimens with equal-sized holes aligned in the tensile loading direction are used. The tensile tests were performed at room temperature with a nominal strain rate of 0.001 s⁻¹. The specimen, containing one hole, showed a significant reduction in ductility compared to the control (no-hole) specimen. With an increasing number of holes, however, the overall strain-to-failure increases and fracture tend to follow shear bands generated locally from the hole edges. Finite element analyses, taking into account the viscoplastic response, were carried out to provide a mechanistic rationale to corroborate the experimental findings. The dispersion of plastic deformation and the effect of hole interaction are both found to contribute to the observed behavior. The local maximum equivalent plastic strain decreases with increasing number of holes, resulting in more delayed fracture. Plastic deformation becomes more intense inside the shear band when the holes are spaced more closely, which explains the increasing propensity of fracture along the shear bands in specimens containing more pre-machined holes.     

Contact behavior of idealized granules bonded in two different interparticle distances: An experimental investigation

This paper presents an experimental investigation on contact behavior of idealized granules bonded in two different interparticle distances, which can be used in discrete element modelling of natural sands featured with interparticle cementation. Firstly, by using the designed specimen preparation devices, two aluminum rods are glued together by adhesive material in two different pre-defined modes, namely thin bond mode and thick bond mode representing different bond thickness between particles. Then, by employing the novel auxiliary loading devices, the mechanical behavior of contact between the bonded rods is obtained while different kinds of forces (i.e., normal force, shear force and moment) are applied in different ways. The experimental results show that both the tension strength and ductility increase with the increasing of bond thickness. However, the force–displacement relationship in compression is characterized with strain hardening in the thin bond mode but strain softening in the thick bond mode. In addition, the peak shear strength and peak rolling resistance increase with the increasing of normal force in the thin bond mode, while they increase with the normal force at first, and then decrease in the thick bond mode. Moreover, the strength envelope is an elliptical paraboloid in the thin bond mode but a teardrop in the thick bond mode in the shear force-normal force-moment space.     

Mechanical property of magnesium alloy sheet with hardening deterioration at warm temperatures and its application for failure analysis: Part II – failure analysis

The mechanical properties of a thin AZ31B Mg alloy sheet (with the thickness of 0.5 mm) were characterized for its anisotropy, temperature-dependent hardening (including its deterioration) and strain rate sensitivity based on simple tension test data measured at 100 °C, 150 °C, 200 °C, 250 °C, respectively, in Part I. As for anisotropy, simple tension tests were performed along three (rolling, transverse and in-between) directions to calibrate the Hill1948 yield function. As for temperature-dependent hardening, hardening as well as its deterioration (or softening) behavior observed beyond the uniform elongation limit was numerically characterized based on the inverse calibration method, in which strain rate sensitivity was also considered. The mechanical properties were confirmed to properly predict failure by strain localization for all the simple tension tests involved in the characterization procedure. Ultimately, the mechanical properties characterized in Part I were applied in Part II to analyze the failure by strain localization in the cross-shaped cup drawing tests developed as the benchmark problem for the NUMISHEET2011 conference [1]. The results showed that the mechanical properties with hardening deterioration properly predicted failure, while hardening without deterioration (obtained following the common practice) did not, confirming the importance of including the hardening deterioration in tensile property characterization, especially to predict forming failure by strain localization.     

Failure prediction for a glass/epoxy cruciform specimen under static biaxial loading

Material models were developed to predict the mechanical behavior of glass/epoxy multidirectional laminates under complex stress states. An incremental plane stress analysis was performed, taking into account the anisotropic material non-linearity, separate damage onset conditions and distinct post-failure stiffness degradation rules. Theoretical formulations were implemented in a shell element of the 1st order shear deformation theory. Numerical results were validated via comparison with test data from cruciform specimens subjected to static biaxial tensile loading. Local strain gauge and full-field strain measurements, obtained using the Digital Image Correlation (DIC) technique, corroborated numerical predictions. Improved strength and failure mode results were derived when, in addition to stiffness reduction, compressive strength degradation in the fiber direction was also considered.     

The response of a multi-directional composite laminate to through-thickness loading

The through-thickness mechanical response of a carbon fibre/epoxy laminated composite of lay-up [0/45/−45]_ns is measured at low rates of strain. Uniaxial tension and compression experiments are carried out on dogbone specimens cut from a thick laminate along different directions, and failure mechanisms are observed via optical and electron microscopy. The effect of direct and shear stresses at the ply interfaces on the onset of failure is measured, and a failure envelope is constructed. The compressive response of specimens of different shape is investigated. Composite beams of different volume and aspect ratios are tested to failure in three-point bending and these tests reveal a strong dependence of the apparent out-of-plane tensile strength of the composite on the beam volume; this effect is modelled by Weibull theory.     

Influence of stress condition on adiabatic shear localization of tungsten heavy alloys

Dynamic deformation and failure behavior of a tungsten heavy alloy (93W) under complex stress condition are studied using a split Hopkinson pressure bar (SHPB) apparatus. Cylindrical, step-cylindrical and truncated-conic specimens are used to generate different stress condition in an attempt to induce strain localization in the alloy. The microstructure of the specimens after tests is examined by optical microscopy and scanning electronic microscopy (SEM). It is found that in all the specimens, except the cylindrical ones, intense strain localization in the form of shear bands is initiated at stress concentration sites. In order to analyze the stress condition of different specimen geometry, finite element simulations are also presented. The Johnson-Cook model is employed to simulate the thermo-viscoplastic response of the material. It is found that dynamic deformation and failure modes are strongly dependent on the geometry of the specimens. The stress condition controlled by specimen geometry has significant influence on the tendency for shear band formation. The adiabatic shear band has general trends to initiate and propagate along the direction of maximum shear stress. It is suggested that further studies on the control of the stress condition to promote shear band formation be conducted in order to improve the penetration performance of the tungsten heavy alloy.     

锈胀开裂钢筋混凝土粘结疲劳性能试验研究

重复加载和锈胀开裂均会导致钢筋混凝土粘结性能退化,进而对钢筋混凝土构件力学性能产生不利影响。该文开展了一系列偏心拔出试验,研究了重复加载以及锈胀开裂对粘结滑移性能的耦合影响规律。主要研究变量包括重复加载次数、应力水平以及钢筋锈蚀程度。结果表明:重复加载对非锈蚀试件和锈胀开裂试件粘结强度及峰值滑移没有显著影响,但会导致钢筋和混凝土之间不断累积残余滑移。重复加载后,粘结应力-滑移曲线形态特征与单调加载试件相似。该文还发现表面锈胀裂缝宽度对粘结强度、峰值滑移以及残余滑移的增长规律有明显影响。锈胀开裂会导致钢筋混凝土试件粘结疲劳寿命显著下降。基于试验数据及文献中研究结论,该文建立了重复及单调荷载作用下非锈蚀及锈胀开裂试件的局部粘结应力-滑移本构关系模型,推导得到了粘结疲劳寿命预测模型。     

Comparative strengths of fresh water ice

In order to gain a quantitative comparison among the various mechanical properties of ice, a series of tests was performed on fresh water ice in which several of the mechanical properties were tested under identical experimental conditions. In particular, the tests were standardized with respect to ice growth conditions, ice type, grain size and orientation, sample size, test temperature and strain rate. The average results of the test were: flexural strength ? cantilever beam = 770 ± 150 kPa; flexural strength ? simple beam (top tension) = 2200 ± 320 kPa; flexural strength ? simple beam (bottom tension) = 1770 ± 190 kPa; $\text{fracture toughness}\text{= 83 ± 7}\text{kPa m}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$; shear strength = 500 ± 220 kPa; compressive strength (horizontal loading) = 4400 ± 700 kPa and strain modulus ? cantilever beam = 1.6 ± 0.4 GPa. Since most of the tests were performed using a portable compression test machine, the strength values can serve as baseline values for comparison with field results.     

Tension stiffening in textile-reinforced concrete under high speed tensile loads

Damage mechanism in glass textile-reinforced concrete (TRC) with and without the addition of Alkali resistant short glass fibers under high speed tensile loading was investigated. The high strain rates ranging from 25 to 100 s⁻¹ were achieved using a high speed servo-hydraulic testing machine. Image analysis by means of digital image correlation (DIC) method was used to obtain the evolution of crack width which was subsequently correlated with stress response. The non-uniform strain distribution was characterized as three distinct response zones of localization, shear lag, and uniform strain and quantitatively measured in each zone. Mechanism corresponding to the basic aspects of tension stiffening modeling were identified by computing the average stress in the matrix phase between two cracks. The width of crack localization zone as well as crack spacing were also obtained using DIC as indications of bonding properties. A finite difference method simulating tension stiffening behavior was employed to predict crack spacing and stress–strain responses of TRC systems. Improvements in bond properties and mitigation of cracking with the addition of short fibers were verified using multiple methods.     

In-situ pressuremeter test in warm and ice-rich permafrost

In order to investigate the in-situ mechanical behavior of warm and ice-rich frozen soils, a series of pressuremeter tests were carried out in permafrost regions on the Qinghai-Tibetan Plateau. Based on the test results, the relationship between stress and strain was obtained using Ladanyi's theory, and it can be described by a hyperbolic model. Moreover, two critical mechanical parameters, ultimate shear strength and initial shear modulus, for each test were deduced from the hyperbolic model. The shear strength increases linearly with decreasing temperature regardless of the water content, while the variation of the shear strength with the water content presents an exponential tendency. Comparing the results of pressuremeter tests with the results of triaxial test and uniaxial test, it can be found that the strengths obtained from pressuremeter tests are always greater than those of indoor tests. In the earlier stage of the pressuremeter test, the circumferential stress is reduced by the same increment as the radial stress increases. However, when the radial stress reaches a certain value, the circumferential stress increases gradually and even becomes a compressive stress.     

Mechanical property of magnesium alloy sheet with hardening deterioration at warm temperatures and its application for failure analysis: Part I - property characterization

The mechanical property of a thin AZ31B Mg alloy sheet (with the thickness of 0.5 mm) was characterized for its anisotropy, temperature-dependent hardening (including its deterioration) and strain rate sensitivity based on simple tension test data measured at 100 °C, 150 °C, 200 °C, 250 °C, respectively, in Part I. As for anisotropy, simple tension tests were performed along three (rolling, transverse and in-between) directions to calibrate the Hill1948 yield function. As for temperature-dependent hardening, the common practice is to characterize hardening only up to the uniform elongation limit and to extrapolate the data to cover the range beyond its limit. In this work, hardening as well as its deterioration (or softening) behavior observed beyond the uniform elongation limit was numerically characterized based on the inverse calibration method, in which strain rate sensitivity was also considered. The mechanical properties were confirmed to properly predict failure by strain localization for all the simple tension tests involved in the characterization procedure. Ultimately, the mechanical properties characterized in Part I were applied in Part II to analyze the failure by strain localization in the cross-shaped cup drawing tests developed as the benchmark problem for the NUMISHEET2011 conference [1].     

Determination of Ductile Fracture Parameters of a Dual‐Phase Steel by Optical Measurements

Marciniak–Kuczynski and Nakajima tests of the dual‐phase steel Docol 600DL ( www.ssab.com/ ) have been carried out for a range of stress‐states spanning from uniaxial tension to equi‐biaxial tension. The deformation histories of the specimens have been recorded by digital images, and the displacement and strain fields have been determined by post‐processing the images with digital image correlation software. The fracture characteristics of the material are presented by means of the stress triaxiality, the Lode parameter and the equivalent strain. These parameters are evaluated on the surface of the specimens based on the optical field measurements and assumptions regarding the mechanical behaviour of the material. Additionally the minor versus major principal strains up to fracture are presented. It is found that the material displays a significantly lower ductility in plane‐strain tension than in uniaxial tension and equi‐biaxial tension, and that it, in the tests exposed to local necking, undergoes large strains between the onset of necking and fracture. Fractographs of selected specimens reveal that fracture is due to growth and coalescence of voids that occur in localised areas governed by shear‐band instability.     

Processing and Properties of Tungsten Heavy AUoys with Ni48Al12Fe40 Inteimetallic Matrix

The superior ballistic penetration behavior of Depleted Uranium (DU) alloys compared to W-Ni-Fe heavy alloys (WHA) has been attributed to a self-sharpening behavior in DU where failure occurs along adiabatic shear bands. Since adiabatic shear represents a plastic flow instability condition between competing processes of thermal softening and work-hardening, cracking along these bands will occur more readily if the matrix were to embrittle with increased temperature along these shear bands. While conventional materials exhibit decreasing strength and increasing ductility with increasing temperature, certain Ll₂+f.c.c intermetaHic compounds such as Ni-12Al-40Fe(at.%) exhibit 'anomalous' behavior where sharply decreased ductility and gradually decreasing strength is observed with increasing temperatures. Hence, tungsten heavy alloys that utilize such intermetaHic matrices have potential to fail by adiabatic shear under high strain rate conditions. Heavy alloys with different weight fractions of tungsten and intermetaHic matrix were investigated. A processing sequence was developed for these novel heavy alloys such that the alloys tested consistently exhibited densities in excess of 97.5% of the theoretical. Such heavy alloys exhibit significantly increased flow stress levels compared to conventional heavy alloys and shear localization during dynamic testing. Further, failure along the shear bands occurred almost immediately following initiation of shear localization. These novel heavy alloys exhibit promise to duplicate the self-sharpening behavior of DU alloys.     

全装配式RC楼盖板缝节点拉剪复合受力性能试验研究

为研究分布式连接全装配RC楼盖(DCNPD)板缝节点在拉剪复合作用下的受力性能,进行了12个板缝节点在纯剪和拉剪复合作用下受力性能试验,对板缝节点的承载能力、裂缝模式、破坏形态、位移延性和应变规律等进行了较为系统的研究。结果表明:拉力的存在减小了由剪切作用引起的连接件与相邻混凝土间的承压作用,致使板缝节点抗剪承载力不同程度的降低,并且拉剪比越大,拉力对板缝节点屈服荷载和峰值荷载的削弱作用越明显;拉剪复合作用下,板缝节点较早呈现出非线性受力特征;拉力对板缝节点位移延性有较大的削弱,尤其对于楼盖中高拉剪比区域应采取措施避免节点发生脆性破坏;综合考虑板缝节点的荷载-位移响应、位移延性特征和破坏模式等因素,HPC、CPC和HP-CPC三种节点是较为理想的DCNPD板缝节点形式,SPC锚筋与锚板间焊缝在拉剪复合作用下易发生断裂,需改进措施以提高其拉剪复合受力性能。